If it is clear tonight when it gets dark, and you’re in northern latitudes, then go outside and look to the south-east. A bright Sun-like star that is easily visible without optical aid has become particularly interesting because it appears to have a solar system of its own.
Tau Ceti has long been a science fiction favourite for planets, featuring in episodes of Star Trek as well as classic works by writers such as Isaac Asimov, Arthur C Clarke and Robert Heinlein.
Now an international team of astronomers has discovered that the star may host five planets orbiting around it, including one in the so-called habitable zone.
Tau Ceti is one of the closest stars to the Earth at a distance of only 12 light-years which will make its planetary system a leading candidate to examine for any signs of life. In fact it has already been a prime target for the Search for Extra-Terrestrial Intelligence (SETI) team before this new discovery was made.
The astronomers, from the USA, UK, Chile and Australia, combined more than 6,000 observations using spectrographs on three giant telescopes – the 3.6-metre telescope at the European Southern Observatory in La Silla, Chile, the Anglo-Australian Telescope in Siding Spring, Australia, and the 10-metre Keck telescope on Mauna Kea, Hawaii. They then used a computer-modelling technique to pick out signals from smaller worlds than can usually be detected.
This allowed them to conclude that there are five planets in the Tau Ceti system with masses between two and six times that of the Earth. One which lies in the zone where water could exist in a liquid state is about five times more massive than Earth. That makes it the smallest planet yet found in the habitable zone of a star like the Sun.
More than 800 extrasolar planets have been discovered in the last 20 years, the bulk of them huge gaseous worlds dubbed “hot Jupiters”. But nearby stars are of special interest because smaller worlds in orbit around them will be easier to find.
The latest discovery is being announced in a paper accepted for publication in the journal Astronomy & Astrophysics, and comes two months after it was revealed that an Earth-sized planet is in orbit around Alpha Centauri B, a star in the closest star system to Earth, just 4.3 light-years away.
The new study was led by Mikko Tuomi of the UK’s University of Hertfordshire. He said: “We pioneered new data modelling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches. This significantly improved our noise modelling techniques and increased our ability to find low mass planets.”
Steve Vogt, of the University of California, Santa Cruz, said “This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets. They are everywhere, even right next door!”
Chris Tinney, of the University of New South Wales, Australia, said: “As we stare at the night sky, it is worth contemplating that there may well be more planets out there than there are star, some fraction of which may well be habitable.”
James Jenkins, of the University of Chile and Visiting Fellow at the University of Hertfordshire, added: “Tau Ceti is one of our nearest cosmic neighbours and so bright that we may be able to study the atmospheres of these planets in the not too distant future. Planetary systems found around nearby stars close to our Sun indicate that these systems are common in our Milky Way galaxy.”